There is a continuing need for porous relatively inert and dimensionally stable materials which can be easily formed into suitable structures for a myriad of utilities. Such materials can be used for filtration purposes, particularly in high temperature or corrosive atmospheres, for the filtration of molten metals such as aluminum or copper, as host substrates for catalysts or reactants in a chemical process, as host substrates for dopants or a diversity of other utilities. Typically, it is appropriate to form the material into a specific convenient size or shape and the ease with which the material can be so formed is an important factor to the commercial acceptability of such material.
One method of obtaining porous ceramic materials has been disclosed in the prior art, as represented by Schwartzwalder et al. U.S. Pat. No. 3,090,094 and Holland U.S. Pat. No. 3,097,930 wherein porous ceramic articles are prepared by immersing an open-celled porous element of pliable synthetic or natural organic material in a slurry of finely divided ceramic powder and ceramic binder, removing the excess slurry from the element and firing the material to burn away the synthetic or organic material while vitrifying the ceramic material. Amongst the various materials listed by Holland are carbides. The successful use of materials prepared in the aforesaid manner in technically exacting functions such as the filtration of molten metals, as catalyst host substrates or as dopant host substrates requires that the material possess particular physical and chemical properties such as superior permeability, structural uniformity, strength and relative inertness to chemical attack.
In the attainment of particular physical and chemical properties the prior art has taken various directions. Wood et al. U.S. Pat. No. 3,833,386 McGaham et al. U.S. Pat. No. 3,175,918 and Googin et al. U.S. Pat. No. 3,345,440 seek to attain superior porous materials by admixing aqueous slurry of particulate ceramic material with polymer reactants and in-situ forming a foamed polymeric/ceramic material. Thereafter, the polymeric/ceramic material is cured and subsequently heated to high temperatures to destroy the polymer, leaving a formed refractory material. Example III of the Googin et al. reference discloses the preparation of a silicon carbide foam by admixing a slurry of silicon with polymer reactants to in-situ produce a foamed polyurethane/silicon material, heating to decompose the polyurethane and thereafter firing to 2200.degree. C. to react the carbon with the silicon to form in-situ silicon carbide. McGaham et al. admixes silicon carbide grit with a resin binder and pore forming material to in-situ form a foamed mix which is cured and heated to carbonize the resin. Provision is also made for the addition of silicon to react with the carbonized resin to form a silicon carbide body. Though the aforesaid in-situ processes have some commercial utility, the methods require an extensive array of apparatus, with the particulate matter significantly complicating mixing and the achievement of uniform porosity.
Other prior art has taken the general direction of first forming a porous body of organic foam material, e.g. such as polyethylene, polyester etc., then impregnating with a slurry of finely divided ceramic material, usually in aqueous suspension, then drying and firing the so obtained structure to decompose the organic foam and create a ceramic structure. Ravault U.S. Pat. Nos. 3,845,181, 3,907,579 and 4,004,933 describe typical procedures utilized in the treatment of various organic foams with aqueous ceramic powder containing suspensions. Therein, treated and untreated foam is impregnated with a slurry of ceramic material which is thereafter dried and fired to form the final porous ceramic article. Yarwood U.S. Pat. No. 4,075,303 improves on the process by utilizing a combined rolling/impregnation step to assure an appropriate final structure.